(a) Field of the Invention
The present invention relates to a stator structure of a motor and, more particularly, to a stator structure easy to create starting offset for the motor.
(b) Description of the Related Art
In conventional designs for direct current brushless electric motors, starting offset must be created to eliminate the dead angle and thus to provide sufficient starting torque for all relative orientations between the stator and the rotor of the motor.
An conventional way of creating the starting offset is to have silicon steel sheets that constitute a stator structure be applied with uneven magnetic force. Referring to FIG. 1, one of the opposite sides of the pole tooth 104 is cut at its periphery to form a tangential plane, so that the gap D, measured from the tangential plane to the rotor 102, is larger than the gap d, measured from the side surface of the pole tooth 104 without the tangential plane, to result in different values of magnetic resistance. Accordingly, the starting offset is introduced by the asymmetry of the pole tooth 104. Except the formation of the tangential plane shown in FIG. 2A, in conventional method the asymmetry of the pole tooth 104 may be achieved by the formation of cambers with different curvatures at two sides, as in FIG. 2B, or the formation of a step portion at one side, as in FIG. 2C.
However, according to the conventional methods given above, the interval between the stator and the rotor are thus enlarged to lower the magnetic flux therebetween (the magnetic flux is in inverse proportion to the square of the distance) so that the torque of the motor is decreased. Also, the asymmetry of the pole tooth 104 is liable to be ill-formed to result in an inaccurate starting offset.
Further, when the starting offset is needs to be adapted to a different rotational sped, load, or threshold voltage, the silicon steel sheet must be replaced with a new one having a different interval between the stator and the rotor. This is costly and time-wasting.